In crown and bridge prosthodontics, a wide diversification of retainers and pontics can be used in various combinations for constructing a bridge. A ceramic to metal restoration uses a framework of metal as reinforcement for the crown and bridge upon which is applied a fired-on coating of a ceramic material such as porcelain. The framework of metal may either be cast or formed from prefabricated units of preformed copings and pontics. In accordance with present practice, a framework may be altered by soldering, but otherwise cannot be modified or reinforced without involving investment and casting operations. Present practice is limited because of the unavailability of commercial materials with which to build up or extend the framework. To reinforce a framework without investment and casting requires adding material to the framework which upon heat treatment will become an integral part of the frame work. The material must be capable of being molded into a desired shape and must be self-supporting in the molded configuration as well as capable of retaining the shape in which it was molded under heat treatment. To be able to shape the material into a desired configuration, the material should be relatively soft and workable. Under heat treatment, the material should solidify into a rigid mass of metal without losing the shape in which it was molded prior to heat treatment. The material should fuse to the metal framework and should have a hardness characteristic of at least equal but preferably greater than the hardness of the material before heat treatment.
Such a material could be used, for example, to build up a cervical shoulder around a retaining member at the gingival margin without the need for investment or casting. For example, a finishing shoulder can be formed around a prefabricated metal coping which was preformed without a shoulder margin. The finishing shoulder can be molded into any shape by the dental technician. Likewise, the material can be used to build metal cusps upon a metal coping before ceramic porcelain is added to provide buccal and/or lingual cusp reinforcement. The material may also be used to strengthen joints at predetermined locations in the framework or for general bridgework repair. The latter is, at present, relatively impossible. Heretofore, the dentist and dental technical were essentially limited to use of cast dental structures and to materials useful as solders or fluxes. Neither the conventional solder nor flux is capable of being molded into a self-supporting configuration nor is either material capable of retaining a shape under heat treatment. Soldering alloys are, in fact, designed to melt and flow freely under the heat of a soldering flame and function to join metals by fusion. A flux is a non-oxidizing agent.
Although no dental material is presently commercially available for reinforcing a dental restoration, there have been attempts in the past to form such a material. All such attempts are based on using a composition which solidifies into a solid mass upon being heat treated and suffers from substantial shrinkage.
In Applicant's parent application, U.S. Ser. No. 176,084, of which this is a continuation-in-part, a dental material composed of metal particles is disclosed for forming, repairing or reinforcing a dental restoration. The composition of metal particles are loose granular particles, preferably held together with a binder, to form a paste or putty constituency which facilitates using the composition as a build up material for reinforcing the framework of a dental restoration. The material is intended to be applied to a metal retainer, shaped into a desired configuration and heat treated. A porous sponge-like structure is formed as a result of the heat treatment having the shape it was given prior to heat treatment. If desired, a low melting temperature filler material may be melted into the sponge-like structure to form an integral solid mass.
Under certain circumstances it is difficult, impractical or undesirable to build up the dental material into the final shape with exacting detail prior to heat treatment. This is due to the difficulty in carving the material to form curves and shapes having accurate contours with any detailed precision. Instead, it is preferable to modify or reshape the material after it is heat treated, either by grinding, pressing or burnishing. The porous structure formed heretofore was found to be vulnerable to breakage and/or to chipping or flaking during any post-cold working operation.
It has been discovered in accordance with the present invention that by the judicious selection of the particle composition of the components in forming the dental material, and the volume relationship between components based upon their specific gravities, a porous structure with a desired void volume may be formed which, upon heat treatment, can be readily reshaped with a minimal degree of susceptibility to breakage or chipping.